A current collector, which is used in devices such as secondary batteries and capacitors, is configured by affixing an electrode active material to the surface of a metal foil.
For example, in a negative-electrode current collector of a lithium-ion secondary battery, a coating layer is formed by applying a slurry—which is prepared by mixing and dispersing a negative-electrode active material composed of a carbon powder, a binding agent, an electrically conductive agent, and the like into a solvent—to a metal foil, and subsequently removing the solvent by drying the coating layer. Subsequently, a pressure-bonding process is performed, as needed, to increase the intralayer density of the negative-electrode active material. In so doing, a negative-electrode current collector having an active-material-containing layer on the surface of a metal foil can be prepared.
The adherence of the active material to the metal foil in the current collector greatly affects the performance of a device. That is, if the active material were to come off of the metal foil, it could lead to problems, such as a decrease in capacity, a decrease in charging-discharging characteristics, a short circuit between the electrodes, or the like, and thereby become a cause that reduces the performance of the device. In addition, if the active material comes off of the metal foil easily, then there is a risk that it will become difficult to maintain the performance of the device over the long term.
As a method that increases the adherence of an active material to a metal foil, a method that roughens the surface of the metal foil is known. For example, Patent Document 1 discloses an aluminum foil for a current collector in which, as the roughness of at least one surface, the mean roughness Ra according to JIS B 0601:1994 is 0.3 μm or more and 1.5 μm or less and the maximum height Ry is 0.5 μm or more and 5.0 μm or less.